libmsc: remove dead code in sms_route_mt_sms()

The following branch:

	if (!rc && !gsms->receiver)
		rc = GSM411_RP_CAUSE_MO_NUM_UNASSIGNED;

at the end of sms_route_mt_sms() always evaluates false.

Just a bit before, in such function, we have this:

        if (!gsms->receiver) {
                ...
 #ifdef BUILD_SMPP
                ...
 #else
                ...
 #endif
                return rc;
        }

So, if there is no receiver, we just stop running code and return the RP
cause via the rc variable. Same applies to the smpp_first check under
the BUILD_SMPP ifdef (that I have removed in this snippet to keep this
commit message small).

Change-Id: Ic3502b5b169bc7a73a67fd6ff53d8b6c0dc045c8

src/libmsc/gsm_04_11.c

@@ -358,9 +358,6 @@ try_local:
 		break;
 	}
 
-	if (!rc && !gsms->receiver)
-		rc = GSM411_RP_CAUSE_MO_NUM_UNASSIGNED;
-
 	return rc;
 }